This invention generally relates to inkjet printing, and is specifically concerned with an apparatus and method for continuously displacing the trajectories of droplets ejected from an inkjet printhead toward a relatively moving receiver so that droplets intended for a particular location on the receiver land on top of one another.
There are two types of inkjet printers, including drop-on-demand printers in which the printhead nozzles eject droplets only when it is desired to print ink onto a receiver, and continuous inkjet printers in which the printhead nozzles eject droplets continuously, the droplets not desired to be printed being captured by a gutter. Both methods are currently practiced.
In drop-on-demand printers, the printhead 1 typically includes a linear row of nozzles 3 which is scanned across a stationary receiver 5 in a fast scan direction 7 as shown in PRIOR ART FIG. 1a. Commercially available desktop printers, for example those made by Epson, operate in this manner. After each fast scan the printhead moves in a slow scan direction 9 relative to the receiver, the slow scan direction being orthogonal to the fast scan direction. Typically, the receiver is moved in the slow scan direction 9 rather than the printhead to effect the relative movement, and another row of printing is completed as is indicated in phantom.
In continuous inkjet printers, the receiver is often moved in the fast scan direction rather than the printhead due to the size and complexity of the printhead. In many cases, the printhead is pagewide and extends across the entire width of the paper to obviate the need for a second scanning movement. The fast scan motion of the printhead relative to the receiver is typically parallel to the length of the printhead.
Drop-on-demand and continuous inkjet printers print droplets on a regularly spaced grid of printing locations or pixels on a receiver, typically at a density of from a few hundred to more than two thousand pixels per inch. Both types of inkjet printers may operate in either a binary (black and white) mode of printing or a contone (also referred to as grayscale) mode of printing. In the binary mode, either a single droplet of a fixed size is printed at each pixel or no droplet is printed. In the contone mode of printing, the amount of ink printed onto a given pixel can be varied over a range of sizes or levels, for example, 10 or more levels. One method to vary the amount of ink printed at each pixel is in contone printing to eject droplets of differing size. However, such an approach is well known in the art to be difficult if substantial variations in droplet size are required, which is usually the case in contone printing. Another method is to print more than one droplet of a fixed size at a given pixel at different times. For example, a second droplet may be printed on a subsequent fast scan pass. This method greatly slows the printing process, especially if substantial variations in the amount of ink per pixel are required. A third more widely practiced method is to eject all of the droplets required at a given pixel during a single scan pass print in rapid sequence so that the droplets print at substantially the same time. In some cases this has been achieved by arranging for each group of sequentially ejected droplets to combine together before landing on the receiver. However, droplets which combine before landing on the receiver may not land at exactly the desired position, since they have been ejected over a range of times. Also the combined droplet may not be spherical when it lands, resulting in image artifacts. In other printers, a group of droplets is sequentially ejected so that the droplets land on the same pixel on the receiver. However, if the receiver is moving quickly relative to the printhead (as desired to achieve high productivity) the droplets landing in a group may be printed as an elongated group that is smeared on the pixel in the direction of receiver motion. Such an elongation within the printed pixel also produces image artifacts and lowers image quality.
To overcome these problems, U.S. Pat. No. 6,089,692, issued to Anagnostopoulos on Aug. 8, 1997, discloses a contone printing method wherein the motion of the receiver is modulated with respect to the printhead by rapidly starting and stopping the receiver in the fast scan direction. This method advantageously allows sequential droplets ejected in a group to be printed at an identical location, thus avoiding pixel smearing. Preferably, the printhead ejects a sequence of equally sized droplets that do not combine before landing on the receiver. During printing of a group of droplets, the receiver motion with respect to the printhead is effectively stopped, and the receiver is moved before the next droplet or group of droplets is printed. Unfortunately, this method requires expensive and precise mechanical controls and hence adds to the cost of the printer and additionally may reduce printer speed due to the time required to accelerate and decelerate heavy components. It is, of course, possible to accelerate the printhead relative to the receiver. But if this is attempted, the printhead may perform poorly due to fluid acceleration and consequent pressure differentials in the ink along the length of the printhead. This is particularly true for pagewide printheads because of the long fluid channels that are distributed over the entire length of the printhead, especially if the displacement occurs rapidly.
Clearly, there is a need for an improved method for contone printing in which a printhead ejects groups of identically sized droplets that land at a single location on the receiver in order to achieve high image quality at no expense to productivity. It would be desirable if such a method could be achieved without the need for expensive and precise mechanical controls that modulate relative movement between the printhead and receiver. Ideally, such a method should be applicable to both drop-on-demand and continuous stream printers. In the case of continuous stream printers, such a method should be achieved without the need for adding any new and expensive droplet steering mechanisms to the printer.
The present invention includes both an apparatus and method for contone inkjet printing using printheads which eject groups of identically sized ink droplets intended to be printed together at a single printing location or pixel. In accordance with the present invention, droplets in such a group land at a single location on the receiver despite the fact that the receiver moves uniformly with respect to the printhead. The trajectories of droplets ejected sequentially in the group are continuously altered so that droplets ejected later in time travel further in the direction of motion of the receiver than do droplets ejected earlier in time. Such trajectory alteration is accomplished by means of the same droplet deflector that is used to separate printing from non-printing droplets. The droplet deflector generates a flow of gas that impinges on the droplet stream comprised of larger and smaller droplets to deflect the larger droplets away from a gutter that captures and recycles the smaller droplets. A controller varies the speed of the deflecting gas flow to further deflect the trajectories of the larger droplets intended for printing so that the droplets intended for a particular pixel land on top of one another despite continuous relative movement between the printhead and the receiver. The apparatus and method are useful in reducing image artifacts and improving image quality and productivity.
While the preferred application of the invention is in a continuous stream inkjet printer, the invention may also be used in a drop-on-demand type inkjet printer.
The droplet deflector includes a tube having an outlet for directing a gas flow into trajectory-altering impingement with the droplets. In one embodiment of the invention, the controller includes a gas flow restrictor for varying the gas flow velocity exiting the tube outlet by variably restricting the gas flow through the tube. The gas flow restrictor may take the form of an expandable bladder disposed within the tube interior. Alternatively, the gas flow restrictor may include a plurality of movable cantilevers, which are either electrostatically or thermally controlled via bimetallic elements that are mounted around the inner surface of the tube. In still another embodiment, the gas flow restrictor may include a plurality of movable vanes disposed within the tube, which restrict more or less of the gas flow in the same manner as venetian blinds.
In still other embodiments of the invention, the controller may include a pressure pulse generator for varying the gas flow velocity in the deflector tube. The pressure pulse generator may include a speaker-like diaphragm in communication with the tube that is connected to an armature which rapidly moved by a piezoelectric transducer. In still another embodiment, the pressure pulse generator may include a diffuser disposed within the tube in combination with a vibrational mechanism that variably vibrates the tube and diffuser toward and away from the droplet stream to create pressure waves within the tube.
In still another group of embodiments, the controller may include an oscillating mechanism for variably oscillating the outlet of the tube with respect to the droplet stream. The direction of the oscillations may be perpendicular to a longitudinal axis of the tube. Alternatively, the oscillations may be in a pivotal direction around a point on the longitudinal axis of the tube.
In all cases, the controller varies the degree of trajectory deflection for the droplets in the stream such that droplets intended for printing on a selected pixel on the receiver are deposited substantially on top of one another despite relative movement between the printhead and the receiver.